1. Field of the Invention
This invention relates to microporous film having improved water permeability and/or reduced electrical resistance.
2. Summary of the Prior Art
Recent developments in the area of open celled microporous polymeric films, exemplified by U.S. Pat. Nos. 3,839,516; 3,801,404; 3,679,538; 3,558,764; and 3,426,754, have instigated studies to discover applications which could exploit the unique properties of these new films. Such films which are in effect a gas-breathing water barrier can be used as vents, gas-liquid transfer mediums, battery separators and a variety of other uses.
One disadvantage of these films, which in the past has limited the number of applications to which they may be put, has been their hydrophobic nature. This is especially true when polyolefinic films, a preferred type of polymeric material often employed in the manufacture of microporous films, are employed. Because these films are not "wetted" with water and aqueous solutions they could not be used advantageously in such logical applications as filter media electrochemical separator components and the like.
Several proposals have been put forth in the past to overcome these problems such as exemplified by U.S. Pat. Nos. 3,853,601; 3,231,530; 3,215,486; and Canadian Pat. No. 981,991, which utilize a variety of hydrophilic coating agents or impregnants. Such coating agents or impregnants, although effective for a limited period of time, tend to be removed in a relatively short time by solutions which contact the films or fibers in which they are present.
Others have attempted to impart a hydrophilic character to a normally hydrophobic microporous film by the use of low energy plasma treatments. Such plasma treatments are achieved by first activating surface sites of the microporous film using argon or hydrogen plasma, and then grafting thereto an appropriate free radical polymerizing species, such as acrylic acid. The plasma treatments result in a film having only a surface which is re-wettable. The surface of the film also becomes plugged when wet which then inhibits or prevents the free flow of water through the interior of the film.
The unavoidable plugging of the surface pores renders the film unsuitable for certain filter applications, increases the electrical resistance of the film, and reduces the dimensional stability of the films as exhibited by substantial shrinkage on drying.
As stated above, a disadvantage of the plasma treatment is its limited ability to render only the surface of the microporous film wettable. It has been observed that due to the unusually large surface area of a microporous film of the type described herein, mere surface wettability does not insure that the film will exhibit certain functional properties such as low electrical resistance, and water flow rates through the film which are comparable to known surfactant systems discussed above.
The primary disadvantage of the plasma treatments, namely, pore plugging and mere surface wettability, are believed to result from a combination of factors, such as the tendency of the low energy plasma to be readily deactivated by the high surface area of the microporous film. This reduces the likelihood that an interior site within the microporous film will be activated. Similarly there is competition for incoming graftable monomers exerted by the free radial polymer grafts which are initially generated at the surface of the film when the graftable monomer first contacts the plasma activated microporous film surface. Thus, the graft polymer chains initially present on the film surface propagate at an increasingly faster rate as the reaction proceeds. Consequently, the resulting lengthened graft polymer chains which occur at the surface of the film entangle and plug the surface micropores in the presence of water.
Other attempts to provide hydrophilic films using a plasma treatment are illustrated by U.S. Pat. Nos. 3,992,495 and 4,046,843.
Another technique for rendering polyethylene films wettable and suitable for use in electrical battery separators is illustrated by V. D'Agostino an J. Lee, Manufacturing Methods For High Performance Grafted-Polyethylene Battery Separators, U.S. National Technical Information Service, A.D. Report No. 745,571 (1972) summarized in 78 Chemical Abstract 5031f (1973); V. D'Agostino and J. Lee, Low Temperature Alkaline Battery Separators, 27 Power Sources Symp. 87-91 (1976), summarized in 86 Chemical Abstract 158277f; 19 V. D'Agostino, J. Lee, and G. Orban, Zinc-Silver Oxide Batteries, (A. Fleischer and J. Lander ed, 1971).
Such articles discuss or relate to a commercial product known as PERMION.TM. developed by RAI Research Corp. Briefly, the method of preparation of this material consists of crosslinking a one mil polyethylene sheet using beta radiation, followed by grafting with methacrylic acid in an appropriate solution under Co60 gamma radiation. The grafted material is washed to remove the homopolymer, then converted to the salt form in hot KOH, washed again to remove the residual base, dried and packaged. The initial crosslinking step creates microcracks or longitudinal slits in non-porous polyethylene film which are so small they are not visible even under an electron microscope. The diameter of these microcracks is estimated to be about 20 Angstroms (10.sup.-8 cm). The microcracks are then grafted with the methacrylic acid. The resulting film is therefore not microporous in the sense of the microporous films employed in the present invention which have an average size of about 100 to about 5,000 Angstroms. The extremely small size of the microcracks of PERMION.TM. generally prohibits a mass transfer of mobile electron carrying species generated by oxidation-reduction reactions through the film at any substantial rate. This is reflected in the relatively high (e.g., 30 to 40 milliohms-in.sup.2) electrical resistances evidenced by films of this type. Moreover, PERMION.TM. is not dimensionally stable in more than one direction as evidenced by substantial swelling.
It is well known that non-porous polymeric substrates such as polyethylene and polypropylene may be reacted with various monomers such as acrylic acid using various types of ionizing radiation as illustrated by U.S. Pat. Nos. 2,999,056; 3,281,263; 3,372,100; and 3,709,718. Since none of these patents are directed to microporous films, however, they are not directed to the peculiar problems associated therewith.
Thus, the search has continued for a relatively permanently wettable, hydrophilic microporous film which exhibits low electrical resistance, and improved water flow rates through the microporous film. The present invention was developed in response to this search.
It is therefore an object of the present invention to provide a process for rendering a normally hydrophobic microporous film relatively permanently hydrophilic thereby improving its water permeability.
It is another object of the present invention to provide a process for reducing the electrical resistance of a normally hydrophobic microporous film.
It is still another object of the present invention to provide a hydrophilic microporous film having reduced electrical resistance.
It is a still further object to overcome the problems of the prior art discussed above.
These and other objects, as well as the scope, nature and utilization of the claimed invention, will be apparent to those skilled in the art from the following detailed description and appended claims.